1. Field of the Invention
This invention relates to a deflection distortion correcting circuit and more particularly to a deflection distortion correcting circuit for easily correcting both linearity of deflection and pairing of scanning lines in cathode ray tube display using interlaced scanning.
2. Prior Art
Cathode ray tube (hereinafter referred to as CRT) displays are widely used as computer terminals. One of the problems related to the deflecting system of CRT display is linearity. This is a phenomenon that deflection per unit deflection angle becomes larger in the periphery of the screen than the center because the radius of curvature of the tube surface is larger than the distance between the center of beam deflection and the tube surface. In order to solve the problem of deflection linearity, S-correction using the LC resonance or cubic curve correction using multipliers has been known. By these correction techniques, deflection linearity can be corrected to a degree that there is no practical problem.
On the other hand, ordinary CRT displays use horizontal raster scanning, and in order to minimize flicker, interlaced scanning is used. Normally, 2:1 interlaced scanning wherein two fields, odd and even, constitute one frame is used. When interlaced scanning is used, another problem was found to arise especially when display with high resolution is constituted.
The anode current of a CRT increases when luminance is high and decreases when luminance is low. Since the output impedance of a high-voltage circuit is large, the anode voltage decreases with the increase of the anode current, and increases with the decrease of the anode current. The change in the anode voltage varies deflection sensitivity. Normally the variation of the anode voltage due to change in the anode current appears relatively slowly due to the action of capacitance of the anode. When interlaced scanning is not used, therefore, the position of the raster is only deviated partly or wholly and there is no practical problem. Especially when the deflection angle is as small as 70.degree.-90.degree., the deviation of position of scanning lines due to change in the anode voltage is insignificant.
When interlaced scanning is used, however, there is normally difference between luminance levels in odd and even fields. This will be understood from the fact that when a character E is displayed on a 7.times.9 dot matrix, the first, fifth and ninth scanning lines are especially bright, leading odd field to have high luminance in this area and even field to have low luminance. When there is difference between the anode voltages in odd and even fields, the distances between odd and even scanning lines adjacent to each other become uneven and the scanning lines approach each other, causing so-called pairing phenomenon. In the worst case, odd and even scanning lines may overlap or change places with each other. The higher the scanning line density and the larger the deflection angle in the vertical direction, the more remarkable pairing appears. Since a larger deflection angle makes deflection sensitivity at the upper and lower parts of the screen larger, pairing is caused by small change in the anode voltage. For instance, when a wide angle CRT with a deflection angle of 114.degree. and a screen size of 17 inches is used with its long axis placed vertically to operate as a full page text display and interlaced scanning is carried out at a high density of about 100 lines/3 cm, pairing is caused unless the fluctuation of the anode voltage is controlled within .+-.0.04%. This value is practically nearly unachievable, and even if a considerable stabilization can be achieved by the use of a high precision voltage regulator or a large capacity power source, it will be very costly and bulky.
It is therefore necessary in high resolution CRT display using interlaced scanning to solve the problem of pairing, and this problem must be solved easily and economically together with the problem of deflection linearity.
Japanese Published Unexamined patent application No. 54-96329 discloses a display unit for minimizing display distortion by the addition of a compensating circuit controlling deflection current in response to the fluctuation of the anode voltage. This prior art suggests the method for controlling the amplitude of the deflection current by supplying a voltage ##EQU1## (K.sub.B :constant) determined as a function of the square root of the anode voltage V.sub.H to a saw tooth wave generator for vertical deflection. However, only the concept is shown and the detailed technique for implementation is not disclosed. Further, no description is given as to how to correct both vertical deflection linearity and anode voltage variation. The problem of pairing is also not recognized.